Group III-nitride heterojunction devices can deliver advantageous properties compared to their silicon and gallium arsenic counterparts for power switch applications. In applications with a high drain bias voltage, however, the group III-nitride heterojunction devices (e.g., in the form of heterojunction field-effect transistors) can suffer from surface-state-induced adverse effects, such as current collapse, due to a channel that is at a short distance (e.g., ˜20 nm or less) from the polarized surface. When electrons are trapped at the surface, they can deplete electrons in the channel. As the HFET is turned on, the trapped electrons cannot be released or emitted fast enough, resulting in a partially depleted channel in the access region between the gate and drain terminals. Consequently, the transient ON-state current becomes smaller than the static ON-state current, and this is the so-called current collapse phenomenon in III-nitride lateral heterojunction devices.
Different passivation techniques have been utilized to reduce the adverse effects in group III-nitride applications. One technique employs SiNx as the passivation material to suppress the current collapse. SiNx passivation layers can be deposited using various chemical vapor deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD). The technique employing the SiNx passivation material has been effective in RF/microwave power amplifiers that typically require a moderate drain bias voltage (e.g., 28 V or 42 V). For power switching at high drain bias (>100 V), however, SiNx-passivation alone is not adequate, and needs to be combined with sophisticated multiple field plates to achieve low current collapse at high drain bias switching.
Another technique employs aluminum nitride (AlN) as the passivation material. Compared to SiNx, the AlN dielectric material exhibits higher breakdown voltage and dielectric constant, larger bandgap, better thermal conductivity and much smaller lattice mismatch to group III-nitrides, such as GaN. Due to these advantageous features, the AlN dielectric material has emerged as a compelling candidate for passivation of group III-nitride heterojunction devices. In situ metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy and sputtering are the three major techniques for growth of the AlN dielectric material on group III-nitride heterojunction devices. In MOCVD and MBE growth, the high growth temperatures (e.g. over 1000 degree Celsius in MOCVD and over 700 degree Celsius in MBE) result in large thermal budget that limits the thickness of high-quality AlN films without cracks. To prepare AlN thin film using sputtering, the possible surface damage induced by the high-energy sputtering ions makes the control of AlN/group III-nitride interface and the consequent passivation a challenging task.
Furthermore, the AlN thin films deposited by sputtering method are amorphous in nature, and thus, do not exhibit obvious spontaneous or piezoelectric polarization. With plasma-enhanced atomic layer deposition technique, it is possible to deposit epitaxial AlN thin film on c-plane III-nitride materials which exhibit strong spontaneous polarization. Thus, the epitaxial AlN thin film also exhibits strong charge polarization. This charge polarization in the AlN thin film is used to compensate the trap states at the interface between the AlN thin film and the surface of a III-nitride heterojunction structure.
The above-described background is merely intended to provide an overview of contextual information regarding group-III nitride heterojunction devices, and is not intended to be exhaustive. Additional context may become apparent upon review of one or more of the various non-limiting embodiments of the following detailed description.